If you need to frequently exchange documents with those who do not have Word 2007, you may want to change the default Word 2007 output type from .docx to .doc.
In a nutshell here's how:
1-2) Choose word options.
3-4) Select Save, Word 97-2003 Document (*.doc)
5) Click OK
Note that if you use Word features unique to Word 2007 you may not want to do this.
For Excel and powerpoint make analogous changes to set default save type to .xls or .ppt